Belts and Gears

I am building a 6' x 6' CNC using 3" x 3" x 0.25" square mild steel
tubing for the purpose of cutting aluminum and steel. I am thinking
of using belts with gears but have no knowledge about how they
perform. What is the backlash or inaccuracy measurements of using
belts & gears as compared with using ball screws, acme screws, and
rack & pinions? Are there belts that minimizes or eliminates the
streching that would occur under heavy load? Is there a difference
between belts that have to gear ridges on them (flat surfaces on both
sides of the belt) and belts with gear ridges on them? Which is more
accurate?

Thanks,